1. Field of the Invention
This invention relates to a method for manufacturing a solid electrolytic capacitor wherein a conductive polymer layer is used a solid electrolyte.
2. Description of the Prior Art
In recent trends toward digitalization of circuits of electric and electronic appliances, there is a strong demand of capicaptors for the circuits which exhibit a low impedance in a high frequency range and is small in size and large in capacitance.
Known high frequency capacitors include, for example, plastic film capacitors, mica capacitors, layer-built ceramic capacitors and the like. Among them the film and mica capacitors are so large in size that a difficulty is involved in attaining a large capacitance. The layer-built ceramic capacitors have been developed in order to meet a demand for a large capacitance and a small size of the capacitors, but temperature characteristics become poor, coupled with another disadvantage that such capacitors become very expensive.
On the other hand, a certain type of capacitor is known as having a large capacitance. Such a capacitor includes, for example, an aluminium dry electrolytic capacitor and an aluminium or tantalum solid electrolytic capacitor.
With aluminium dry electrolytic capacitors, anode and cathode aluminium foils which have been etched, respectively, are convolutely wound through a paper separator and a liquid electrolyte is impregnated in the separator. This type of capacitor has the serious problem that the characteristics deteriorate due to the leakage and/or evaporation of the liquid electrolyte. To avoid this, solid electrolytes are used in the aluminium or tantalum solid electrolytic capacitors.
In the aluminium or tantalum solid electrolytic capacitor, a metallic foil such as an aluminium or tantalum foil which has been anodized to form a dielectric film or oxide film on the surface thereof is immersed in a manganese nitrate aqueous solution. Then, the manganese nitrate is thermally decomposed at a temperature of approximately 350.degree. C., thereby forming a solid electrolyte layer made of manganese dioxide. Since the electrolyte is solid, the drawbacks, such as deterioration of characteristics involved by the effluent of the electrolyte at high temperatures or by coagulation of the electrolyte at low temperatures, can be avoided. These solid electrolytic capacitors exhibit better frequency and temperature characteristics than capacitors using liquid electrolytes. In addition, the oxide film serving as a dielectric material can be made very thin, thus being suitable for fabricating a capacitor with a large capacitance.
Aside from the above-described capacitors, there are other types of solid electrolytic capacitors including capacitors using 7,7,8,8-tetracyanoquinodimethane (TCNQ) complexes instead of the manganese dioxide layer and capacitors using, as the solid electrolyte, a conductive polymer layer formed by electrolytic polymerization of polymerizable monomers such as pyrrole, furan and the like.
The solid electrolytic capacitors using the manganese dioxide layer as the solid electrolyte are disadvantageous in that the dielectric film will be damaged during plural cycles of thermal decomposition treatments. Additionally, the specific resistance of the manganese dioxide layer is so high that the lost (tan .delta.) in a high frequency range is not satisfactory.
The capacitors using the organic semiconductors such as TCNQ complexes have the problem that the specific resistance is increased at the time of application of the organic semiconductor and that the adhesion of the semiconductor to anodized metallic foil is not so high. Thus, such capacitors are not satisfactory with respect to characteristic properties.
On the other hand, with the capacitors using, as the solid electrolyte layer, an electrolytically polymerized conductive layer, the polymer layer is formed by the electrolytic polymerization reaction. Since the electrolytic polymerization reaction proceeds through an electrolytic oxidation reaction of monomers, it is very difficult to form the polymer layer on a metal surface which is insulated with a dielectric film. It may be possible to form the electrolytically polymerized conductive layer prior to the formation of the dielectric film. This will involve a change in quality of the electrolytically polymerized film and a relatively poor adhesion between the polymer film and a metallic film. Thus, it is not of practical utility.
We have proposed in U.S. Pat. No. 4,943,892 that a metallic foil having a dielectric film is formed with a manganese dioxide layer on the film and a conductive polymer layer is formed on the manganese dioxide layer by electrolytic polymerization of a polymerizable compound by contact of an anode electrode with the manganese dioxide layer from outside. The resultant solid electrolytic capacitor has a solid electrolyte layer made of the manganese dioxide layer and the conductive polymer layer.
This solid electrolytic capacitor has good frequency, temperature and life characteristics but is not satisfactory with respect to breakdown voltage. The reason for this is considered as follows.
For the formation of a conductive polymer layer for a solid electrolyte by electrolytic polymerization, when the metallic foil having a dielectric film is used as an electrode, little current is passed, so that the polymerization reaction scarcely proceeds. Accordingly, an electrode for an anode (i.e. an electrode for polymerization reaction) is in contact with the surface of the manganese dioxide layer, to which a potential is applied. The portion at which the electrode has been contacted is apt to be produced with mechanical defects owing to the contact of the electrode, causing dielectric breakdown to occur. Thus, a satisfactory breakdown voltage is not achieved. In addition, the portion at which the electrode has been contacted is liable to be formed with a conductive polymer layer which is smaller in thickness than the other portions. This is considered to be another factor for lowering the breakdown voltage.
Moreover, when the conductive polymer layer is formed on the conductive manganese dioxide layer formed on the dielectric film through the electrode for the polymerization reaction, the potential of the metallic foil isolated with the dielectric film becomes negative, causing the dielectric film to be dissolved out and damaged. This leads to a substantial amount of leakage current with a poor breakdown voltage characteristic.